The present invention relates to an improved structure of a treadmill, more particularly, to a frame structure of an electrical, collapsible treadmill. The frame structure includes a front frame assembly, a rear frame assembly and elevation mechanism disposed under the front frame assembly. The elevation mechanism includes a front leg assembly, a rear leg assembly and an elevation device, of which the front leg assembly and rear leg assembly are pivotally and slidably connected to each other, such that the elevation mechanism is able to support the front and rear frame assembly by means of scissors type supporting structures without the need of a base. When in use, the treadmill can be kept stable without the need of any extra fastening means; whereas when being stored, the treadmill can be folded by only flipping over the rear frame assembly upward. This design makes the convenience in storage and reduces the costs in assembling and manufacturing.